"The cooler air began sweeping southward Sunday in the eastern half of the country, bringing down some temperatures by 15 or more degrees from Saturday's highs, which topped 100 in cities including Philadelphia, Washington, St. Louis, Indianapolis and Louisville, Ky."

"Instead of widespread highs in the 100s like we saw on Saturday from Kansas and Missouri to the Mid-Atlantic, we'll be talking about highs in the 80s and low 90s this week across the same swath of territory."

But, Weather.com also warns that "the West will not be so lucky. Highs in the low 100s will move as far north as Salt Lake City, Utah, Reno, Nev. and Boise, Idaho this week. Las Vegas, Nev. and Phoenix, Ariz. will be 110+ on both Monday and Tuesday."